The invention pertains to a drive system for a utility vehicle having a drive motor that is connected to and drives a hydraulic pump, which, in turn, is connected to a hydraulic motor so that hydraulic fluid is conveyed between the pump and the hydraulic motor. The hydraulic motor is connected to and drives a ground engaging member through a gearbox, which has different selectable gears. A speed-adjustment device cooperates with the hydraulic pump and/or the hydraulic motor is provided for adjusting the rotational speed of the hydraulic motor, and wherein the speed-adjustment device can be adjusted over a range that includes an upper limit in order to define a maximum speed.
Means to limit the maximum speed which the hydraulic pump of a utility vehicle, particularly an agricultural harvesting vehicle, can be driven, are known from the prior art. For this reason, the maximum pivot angle of the adjustment lever of the hydraulic pump is mechanically limited by means of a limit stop. This limit on the utility vehicle is factory set in accordance with the legal stipulations of a given country. For example, the maximum speed for harvesting machines in Germany is 20 km/h, with 25 km/h for other European countries. The setting of the maximum speed takes place in the highest gear usually designed for road travel.
It is disadvantageous that the mechanical limitation on the maximum speed of the hydraulic pump not only limits the road speed of the vehicle, but also the speed for field work in all other gears. The speed of a harvesting machine driven on roads is limited to 20 km/h in Germany. However, in principle, the maximum speed in all gears that are used during field work is unnecessarily limited due to the mechanical limitation on the hydraulic pump adjustment lever pivot angle.
The invention is based on the objective of improving a drive system of the above-described type in such a way that the propulsion is not unnecessarily limited in the lower gears of the gearbox.
The invention proposes to select the range of motion of the speed-adjustment device that cooperates with the hydraulic pump and/or the hydraulic motor as a function of the selected gear of the gearbox. This makes the full range of motion available, especially for the lower gears of the gearbox. Thus, the full rotational speed range of the hydraulic motor and the entire vehicle speed range are also available. However, if the highest gear (or one of the higher gears) is selected in which the maximum permissible speed can be exceeded in a corresponding position of the speed-adjustment device, the range of motion of the speed-adjustment device is defined, or limited, in such a way that the maximum permissible speed cannot be exceeded. In this case, the rotational speed of the hydraulic motor is limited to a fixed value that is below the maximum set value for another gear.
In this way, the full rotational speed range of the hydraulic motor is available in the lower gears, while it is ensured that the maximum permissible speed is observed in the highest gear. This speed cannot be exceeded in the lower gears due to the gear ratios. The speed-adjustment device may be actuated by a drive that is controlled by an electronic controller and moves the speed-adjustment device over a defined adjustment range. In principle, this can be realized in one of two ways:
It is possible to provide a control device that operates without feedback. The respectively selected gear ratio of the gearbox is detected by a sensor. If the gear ratio is so high that the maximum permissible speed can be exceeded in a corresponding position of the speed-adjustment device, the range of motion of the speed-adjustment device is automatically limited to a permissible range. This may be realized mechanically, electronically or electro-mechanically. Otherwise, the full range of motion of the speed-adjustment device is available. The sensor may be linked to the drive lever, the actuation of which is only permitted in the field, which implies that the highest gear ratio is not selected. Alternatively, it would also be possible to utilize a road/field switch or a switch that responds when the highest gear ratio is selected.
It would also be possible to provide a control device that measures the propulsion or a parameter related to it, for example, the rotational speed of the wheels or the rotational speed of the output shaft of the gearbox. Alternatively, the rotational speed of the hydraulic motor and the position of the adjustment lever for selecting the gear ratios of the gearbox may also be measured at the same time. The propulsion can be calculated on the basis of these parameters. The control device takes into consideration a nominal speed value that is predetermined by a drive lever or an accelerator pedal and correspondingly adjusts the speed-adjustment device. If the measured speed is faster than the given permissible maximum propulsion, the control device automatically adjusts the speed-adjustment device to a slower speed in order to limit the speed to the maximum permissible value. Thus feedback is used. It is not absolutely imperative in this case to separately sense the given selected gear ratio because it can be determined from the measured propulsion.
Another possibility consists of providing a limit stop that upwardly limits the range of motion of the speed-adjustment device mechanically when a gear is selected in which the maximum permissible speed can be exceeded. The speed-adjustment device can be actuated mechanically or electro-mechanically. The limit stop is ineffective when another gear is selected.